Disable the related “Redirect” features in “Website for Domain” in the Server Template. (You can also search for “Redirect” in the Server Templates search and it’ll take you to it.)
Also note there is a “Add above to the automatically generated records” and “Use only the records above” option, which allows you to completely replace the automatic records with your own. But, just disabling the features that result in extra names will also do the job.
Disable the “Mail for domain” feature in Features and Plugins, or for just this one domain, if you only want to disable it for this one domain. (Note, this obviously actually disables incoming mail for the domain, not just the name in DNS. The user can still send mail, as any user on the system can, assuming you have setup the system to be able to send mail and you have port 25, but receiving mail for domain users will not be available.)